This invention relates to a core for casting a container, apparatus for casting a container and a method for casting a container.
In particular it relates to a core, apparatus and method for casting a container comprising a core divided into least two core segments.
It is well known in the art to cast hollow components using a mould and core, where the core sits inside the mould and defines the shape of the internal features of the casting. The core may be made from a fired ceramic or a sand and resin mix and consequently the casting alloy tends to have a considerably faster thermal contraction rate to that of the material of the core. For hollow components which surround the core, this results in the component shrinking onto the core, with solidified regions of the casting applying tensile hoop stresses to semi-molten areas. This induced stress can cause the casting to tear or crack before cooling and solidification is complete, and thus before all regions of the casting have gained sufficient strength to resist cracking or tearing.
Cores can hollowed out, and optionally filled with a low-density material such as polystyrene, thereby enabling them to fracture or crumble, and therefore contract along with the casting. However, the walls of the cores can only be thinned so much before they lose structural integrity and are unable to maintain the required shape during the pouring of the molten metal. Hence a balance is required between weakening the core to allow it to collapse during the cooling process and leaving sufficient core material to retain the required shape during the pouring of the molten metal.
Alternatively the wall thickness of the casting can be increased such that cracking/tearing does not occur. Clearly this is undesirable as this either increases overall component weight, or increases the cost of manufacture as additional machining operations are required to remove the excess material to reduce wall thickness to a desired size.
U.S. Pat. No. 3,506,235 (Katz et al.) describes a heat sink core comprising a plurality of segments which are spaced apart and surrounded by a sleeve which fractures when the casting cools. Resilient spacers are provided between the segments. The sleeve blocks the gap between segments but gives way when enough pressure is applied during cooling/contraction of the casting. However, the invention of Katz et al can only be used for the manufacture of annular, cylindrical, smooth sided and open ended components, since the sheath has to fit neatly and evenly around the segmented core.